This invention relates to a method for the coating of continuous tows.
Composite materials are widely known and widely used. By combining a polymer with another material, such as glass, carbon, another polymer, or the like, is it possible to obtain unique combinations or levels of properties. Similarly, by combining a metal or glass with selected fibers, it is possible to obtain unique combinations or levels of properties. Advanced composites have evolved as a class of structural materials as a result of the development of high-modulus, high-strength, low-density reinforcing fibers.
The presence of a carbon interlayer along the fiber-matrix interface has been shown to be responsible for the high toughness and strain to failure of Nicalon.RTM. (SiC fiber)/lithium aluminosilicate glass composites and Nicalon.RTM./Ba-Si-Al oxynitride glass composites. However, these composites are not viable for high temperature oxidizing environments. Such environments require oxidation resistant fibers, matrices and interlayers. One approach to fabricating a fiber-matrix interface is to introduce an interlayer as a fiber coating before the composite is densified. After densification, the interlayer chosen should cause crack deflection and fiber pullout similar to carbon interlayers, or should provide oxidation resistance for other interlayers.
Several types or combinations of interlayers are considered to be feasible, including microporous interlayers, reactive interlayers which lose volume, and interlayers with ductile particles. However, application of a coating, particularly a uniform coating, to continuous fibers and fiber tows can be difficult. Measurement of coating thickness can also be difficult.
Several techniques are known for applying coatings to continuous fibers. Fiber coating may be accomplished by passing the fibers through a container filled with a coating liquid, which container has one or more rollers or wheels to keep the fiber immersed in the liquid while coating. One disadvantage of this process is that the fibers must be bent around the roller(s) or wheel(s) and may sustain damage from bending or abrasion. Another disadvantage is that the fibers may be contaminated from contact with the wheel or roller.
Coatings may also be applied by spraying. The primary disadvantage of this coating method is that spraying is a line of sight process, so coating thickness is dependent upon the angle at which the spray jet contacts the fiber. Other disadvantages are that spray jets tend to clog easily, the characteristics of the jet may change with time, making control of the process difficult, viscous coating solutions are difficult to apply as a spray, and low viscosity solutions tend to run off the fiber before they are cured.
Fibers may be coated by passing same through a container having a gasket which seals around the moving fiber and prevents coating liquid from flowing out. The disadvantages of this method are that the fiber surface may be contaminated or abraded by contact with the gasket, and fibers having irregular cross-sections or multifilament fibers or tows tend to get caught along irregularities or at broken fibers in gaskets tight enough to prevent leakage of the liquid.
The coating of multifilament tow or cloth is particularly vexing. Most methods used to apply liquid-based coatings to monofilaments do not uniformly coat individual filaments of a fiber tow or cloth because the coating cements the filaments together, or forms thin bridges between filaments. There are, however, methods which may be used to coat each filament individually.
If the coating liquid is a acidic sol, it may be applied in dilute concentration, then gelled on the individual filaments by passage through ammonia. The use of an ammonia atmosphere together with dilute acidic sols is a disadvantage.
The tow may be sprayed with a coating liquid. As mentioned previously, spraying is a line of sight process, so coating thickness depends on the angle of contact of the spray jet relative to each filament in the tow. Thus, some filaments may be completely shadowed by other filaments and receive no coating.
The individual filaments in a tow may be uniformly coated by chemical vapor deposition (CVD). Although this technique works well for many different types of tow and coatings, it carries with it several disadvantages, including slow coating rates, equipment expense, precursor expense and its unsuitability for applying complex oxide coatings.
It is an object of the present invention to provide a method for coating continuous tow.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the invention.